More particularly, it relates to a novel material capable of reacting with at least one gaseous aldehyde, preferably formaldehyde, and also to its sol-gel preparation method.
It also relates to a method for detecting and/or quantifying and/or trapping at least one gaseous aldehyde, especially formaldehyde, based on measurements of the variation of at least one physicochemical property of said material.
It lastly relates to the use of these novel materials in optical transduction sensors which can be exploited for a metrology of the aldehydes in an environment, and also to devices enabling pollution control.
The term “aldehyde” denotes any organic molecule having a terminal carbonyl functional group preferably chosen from formaldehyde, acetaldehyde, propionaldehyde, butryaldehyde, acrolein, pentanal, hexanal and benzaldehyde.
When the notion of pollution is raised, it is conventional to refer to the pollution of the air outside. Furthermore, most of the epidemiological surveys carried out to establish correlations between pollution and the appearance of respiratory diseases usually only involve pollutants measured outside of dwellings. However, most people spend most of their time indoors. The quality of the air inside therefore appears particularly important from the point of view of health and well-being.
It is only recently that studies have mentioned the possibility of the role of chemical pollutants of the environment indoors in the increase in the prevalence of respiratory diseases.
Aldehydes are among the most abundant domestic chemical pollutants. Their sources are extremely numerous. These sources may be, in particular, connected to an external production such as the photooxidation of methane. However, the main sources for the release of aldehydes are found inside dwellings and are very diverse:                resins and adhesives used to manufacture chipboard, particle board and plywood;        urea/formaldehyde insulating foams used as thermal insulation, by injection into walls and partitions; and        in textile coverings, wallpaper, paints, leather, etc.        
Formaldehyde is also a preservative, disinfectant and desiccant. For these reasons, it is widely used as a solvent in hospital surroundings for disinfecting surgical instruments and also in the funeral service industry where embalming is carried out.
Formaldehyde is the most studied of aldehydes as it is widely used in the manufacture of very many construction products and various equipment. The release of formaldehyde varies depending on the temperature and humidity conditions. Its pungent odor is detected by a person at low concentrations (from 0.048 to 0.176 ppm or from 0.06 to 0.22 mg/m3). Exposure to formaldehyde causes irritation which is experienced by most of the population at concentrations between 1 and 3 ppm, this irritation being rapidly aggravated when the content rises. Most individuals cannot, in effect, tolerate a prolonged exposure at 4-5 ppm. At 10-20 ppm, signs of severe irritation of the ocular mucous membranes and airways occurs from the start of exposure. Staying, even briefly, in an atmosphere where the formaldehyde concentration is greater than 50 ppm may cause serious disorders of the respiratory system (acute pulmonary edema, tracheal and bronchial ulcers, etc.). Due to long-term risks, formaldehyde has been classified as carcinogenic by the International Agency for Research on Cancer.
Consequently, the French national legislation has developed so that it is now recommended not to exceed a formaldehyde content of 0.2 ppm, or 0.25 mg/m3, in dwellings insulated using urea/formaldehyde foams. Furthermore, the World Health Organization (WHO) recommends that the formaldehyde concentration does not exceed 0.080 ppm, or 0.1 mg/m3, for a 30-minute exposure, this value corresponding to an order of magnitude below that for which there is a risk of harm occurring.
Considering the harmful effects of such chemical pollutants on public health, it appears necessary to measure and control the contents of aldehydes, especially including formaldehyde, in contaminated environments, whether they be outside or inside, and to offer novel pollution-control devices.
The detection methods that are already commercially available are based on trapping aldehydes by reaction with a suitable molecule, then analyzing them by gas or liquid chromatography.
In certain methods, the aldehyde, especially including formaldehyde, is trapped on an absorber or a solid support (silica or octadecyl-grafted silica) impregnated with a reactant such as 2,4-dinitrophenylhydrazine (DNPH) or 2-hydroxymethylpiperidine, capable of reacting with the aldehyde to form a product, a hydrazone or an oxazolidine. For example, the NIOSH 2451 method consists of a take-up of formaldehyde on a solid absorbent impregnated with 2-hydroxymethylpiperidine, followed by a gas chromatography analysis. The detection limits of this method are from 0.01 to 38 ppmv.
Due to the non-specificity of these reactants for formaldehyde, the aforementioned methods only allow a detection of formaldehyde when the trapping step is coupled to a gas or liquid chromatography analysis which makes it possible to differentiate the various reaction products.
Nash was the first to identify a mixture of reactants capable of reacting specifically in solution with formaldehyde. These reactants are a β-diketone, for example acetylacetone and ammonium acetate. They give rise to the formation of a highly fluorescent derivative, 3,5-diacetyl-2,6-dihydrolutidine (DDL) [Nash T., Biochem. J., 55, 416, (1953)]. Sawicki et al. then extended this reaction to other ketones such as dimedone [Sawicki E. et al., Mikrochim. Acta, 148, (1968); Sawicki E. et al., Mikrochim. Acta, 602, (1968)]. In this case, the final product is 3,3,6,6-tetramethyl-1,2,3,4,5,6,7,8,9,10-decahydro-1,8-acridinedione, whose fluorescence is much higher than that of DDL.
By studying the mechanism for the formation of 3,5-diacetyl-2,6-dihydrolutidine, it has been discovered that a reaction intermediate, 4-amino-3-penten-2-one or Fluoral-P, was capable of reacting rapidly and quantitatively with formaldehyde [Compton B. J., Purdy W. C., Can. J. Chem., 58 (1980) 2207-2211]. However, the specificity of Fluoral-P for formaldehyde, in solution, seems dubious since aldehydes of modest size (up to around 10 carbons) are also capable of reacting rapidly with Fluoral-P [Compton, B. R., Purdy, W. C., Anal. Chem. Acta., 119 (1980) 349-357].
Detection methods based on mixed solid/liquid trapping systems and using Fluoral-P have been developed. One of these systems uses methods of injection of Fluoral-P and formaldehyde in a liquid stream followed by retention of the product formed on a grafted silica support of C18 type impregnated with the elution solvent. According to this method, the analysis is carried out by absorbance or by fluorescence [Teixera, L. S. G., et al., Talanta, 64 (2004) 711-715]
A good sensitivity may be obtained by these detection methods. However, they have a drawback of not allowing the direct detection of the aldehydes in gas form.
Recently, it has been shown that 4-amino-4-phenylbut-3-en-2-one could be used in solution as a specific reactant for formaldehyde in order to form a lutidine derivative [Suzuki Y., Nakano N., Suzuki K., Environ. Sci. Technol., (2003), 37, 5695-5700]. The authors used a device comprising a cellulose filter paper covered with silica granules and impregnated with 4-amino-4-phenylbut-3-en-2-one. Colorimetric detection of the lutidine derivative was carried out by reflectance as the filter paper is not transparent. The sensitivity obtained was 5 ppb with a response time of 15 minutes. This method has the disadvantage of being relatively sensitive to the degree of ambient humidity and to temperature. Specifically, the measurements are impaired when the degree of humidity is outside of a range of 30-700, and/or when the temperature exceeds 35° C. Furthermore, after keeping for more than six months, a reduction in the sensitivity of the impregnated paper is observed. The presence of silica granules which have the particularity of attracting and maintaining the humidity by capillary action makes it possible to explain, at least partly, the fact that the degree of ambient humidity influences this method.
WO 2004/10457.3 describes a sensor capable of detecting formaldehyde in an atmosphere, consisting of a generally polysaccharide gel based, for example, on xanthan gum or gum Arabic, pectin, starch, agar or alginic acid, the gel comprising a Schiff base such as pararosaniline, sulfuric acid or one of its salts, another acid to adjust to pH 3 and water.
U.S. Pat. No. 6,235,532 describes a method of detecting 2-furaldehyde in oil using aniline acetate. The detection is carried out using a porous sol-gel, especially methyltrimethoxysilane, matrix containing aniline acetate.
One of the main problems of the methods of the prior art is that they do not allow the direct and in situ detection and/or quantification of formaldehyde or of other aldehydes in gas form irrespective of the conditions of the environment. Certain methods of the prior art require the withdrawal of samples and the trapping of the gas in liquid/solid phase to enable a qualitative and/or quantitative analysis. Other methods of the prior art are highly sensitive to the degree of ambient humidity or to temperature.